This invention relates generally to the field of concrete construction and more particularly to the field of concrete formwork systems and methods utilized to create internal cavities within monolithic concrete structures. More particularly, the invention relates to such systems and methods wherein modular or sectional formwork is used. Even more particularly, the invention relates to such systems and methods wherein the internal cavities are bores, tubes or tunnels of complex configuration having smooth, curved surfaces, such as the spiral cases or draft tubes found in hydro-electric power plants.
It is often necessary to produce internal cavities, such as chambers, bores, tubes, tunnels, passages or the like, within large monolithic concrete structures. In many instances, this is accomplished by constructing a form, the outer surface of which conforms to the desired interior surface of the cavity. Wet concrete is poured around the form, allowed to cure, and the form is removed to expose the cavity. A common method is to construct the form out of wooden components. This method is labor intensive and works best only with relatively simple cavity shapes having planar surfaces or segments of surfaces. Production of a form having complex, curved surfaces, such as a tube or a bore, is difficult and time-consuming, especially where accuracy and surface precision is essential in the finished structure. Dismantling the wooden form is also a difficult and time-consuming task.
Because of the drawbacks inherent in wooden forms, it is also known to create sectional or modular forms out of metal segments. The metal segments are sturdier and can be re-used to form multiple cavities, but must be constructed of relatively small dimension due to the increased weight. Examples of segmented metal forms for the formation of cavities within concrete structures can be seen in U.S. Pat. No. 992,782 to Lambie, U.S. Pat. No. 3,729,165 to Trimble, U.S. Pat. No. 4,997,602 to Trimble, and U.S. Pat. No. 5,032,197 to Trimble, which all disclose metal forms for creating manholes. The use of such methodologies, however, typically produces bore or tubular surfaces with stepped or angled annular junctions joining short sections or segments that are planar in one or both directions. In many situations where optimization and control of aerodynamic or hydrodynamic flow is required, cavities produced by this method are less efficient due to the flow interruptions caused by the junctions.
It is also known to utilize sectional or segmented forms composed of plastic, fiberglass or similar material. Examples are shown in U.S. Pat. No. 3,682,434 to Boenig, which shows external forms joined to receive wet concrete in the interior of the form, and U.S. Pat. No. 4,728,073 to Smith, which shows the use of flexible panels to create reservoir walls. Use of plastic forms is limited due to the lightweight and flexible nature of the material, and bracing or other types of supporting frameworks are necessary for any applications of large dimension. The known systems and methods utilizing plastic forms are not suitable for large concrete structures due to the massive weight of the concrete that must be supported by the forms during the curing process. For example, a spiral case bore for delivering water into a turbine within a hydroelectric power plant may have poured concrete walls four meters or more in thickness surrounding a bore having a diameter of eighteen feet or more.
An alternative sectional system and method wherein metal forms are utilized to define the internal cavity within a large monolithic concrete structure, and in particular as used for creating a large draft tube that receives water from the turbine in a hydroelectric power plant, is disclosed in U.S. Pat. No. 5,108,671 to Chapdelaine. In this system, the sectional forms are composed of pre-formed concrete panels embedded within peripheral metal frames. The metal frames of adjacent panels are joined together by mechanical fasteners. While this produces panels that are very rigid and durable, the material composition insures that they are extremely heavy. Thus, assembly and disassembly of the forms is difficult and the maximum overall dimensions of each panel are severely limited, increasing the number of joints required to construct a form.
It is an object of this invention to provide a system and a method of creating a cavity within a large monolithic concrete structure, in particular wherein the cavity is a tube, duct or bore of curvilinear or irregular shape, such as found in the spiral cases or draft tubes of hydro-power plants, in wind tunnels, in sewage systems, etc. It is an object to provide such a system and method of increased efficiency, where the bore has no undesirable corners or edges and can be formed in an infinite number of shapes, dimensions and transitions. It is a further object to provide such a system and method wherein the cavity is created by assembling, either prefabricated or on-site, sectional elements that can be disassembled, removed and re-used to create additional cavities of identical structure. It is a further object to provide such a system and method such that problems inherent in the previously known systems and methods are obviated. It is a further object to provide such a system and method wherein the cavity is precisely defined, the smoothness of the cavity surface is optimized for hydrodynamic or aerodynamic characteristics, the form is sectional or modular in nature, and the assembly and disassembly of the form is readily accomplished in a time efficient manner. It is a further object to provide such a system and method wherein the sections or panels comprising the form are relatively lightweight and somewhat flexible, each section comprising a relatively low density, lightweight, low compressive strength, slightly flexible, polymer-aggregate concrete member disposed within a flexible, flanged skeleton mold member. These and other objects not expressly set forth above will be made apparent by the disclosure to follow.